Constant-current/constant-voltage circuit architecture

ABSTRACT

Methods and circuits implementing a constant-current/constant-voltage circuit architecture are provided. The methods and circuits preferably provide a charging system that provides current to a load using a fixed current until the load is charged. When the load is charged, the methods and circuits preferably provide a variable current to the load in order to maintain the voltage level across the load. This variable current varies according to the voltage across the load. In one embodiment of the invention, a constant power current may also be used as one of the load charging currents. The constant power current may act as a limit on the charging circuit&#39;s power output.

CROSS-REFERENCE TO A RELATED PATENT APPLICATION

[0001] This application is a continuation of U.S. patent applicationSer. No. 10/443,299, filed May 21, 2003, which is a continuation of U.S.patent application Ser. No. 10/106,499, filed Mar. 27, 2002 (issued asU.S. Pat. No. 6,570,372), which is a further continuation of U.S. patentapplication Ser. No. 09/837,658, filed Apr. 18, 2001 (issued as U.S.Pat. No. 6,522,118).

BACKGROUND OF THE INVENTION

[0002] This invention relates to circuitry and methods which may be usedto provide a current to a load. More particularly this invention relatesto circuitry that provides a constant current to a load until thevoltage across the load reaches a certain value. When this value isreached, the current delivered to the load must be varied to maintain aconstant load voltage.

[0003] This type of circuitry is referred to as aconstant-current/constant-voltage system, and the charging circuitry ofa lithium ion battery is a common use for such circuitry. Frequently,conventional systems use a programmable resistor to set the value of theconstant charging current.

[0004] It would be desirable to provide circuitry that provides aconstant current to a load until the voltage across the load reaches apre-determined value and then maintains the voltage across the load atthe predetermined value by varying the current to the load.

[0005] It would also be desirable to provide a signal proportional tothe load current.

SUMMARY OF THE INVENTION

[0006] It is an object of the invention to provide circuitry thatprovides a constant current to a load until the voltage across the loadreaches a pre-determined value and then maintains the voltage across theload at the predetermined value by varying the current to the load.

[0007] It is also an object of this invention to provide a signalproportional to the load current.

[0008] The circuit according to the invention includes a first currentloop that is adapted to provide a fixed current, a second current loopthat is adapted to provide a variable current, and a priority circuit.

[0009] The priority circuit receives a first signal from the firstcurrent loop and a second signal from a second current loop. The firstsignal indicates the level of an available fixed current. The secondsignal from the second current loop indicates the level of an availablevariable current. The priority circuit may compare the two signals andselect one of the first current loop and the second current loop toprovide current to the load based on a predetermined priority assignedto the first signal and the second signal—e.g., whichever current haslower magnitude.

[0010] A method of charging a load according to the invention includesselecting a load charging current from one of a fixed current and avariable current, the variable current being based on the voltage acrossthe load, the selecting being based on a predetermined priority.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The above and other objects and advantages of the invention willbe apparent upon consideration of the following detailed description,taken in conjunction with the accompanying drawings, in which likereference characters refer to like parts throughout, and in which:

[0012]FIG. 1 is a prior art constant-current charging circuit;

[0013]FIG. 2 is an exemplary constant-voltage charging circuit thatprovides continuous continuous information relating to the magnitude ofthe charging current according to the invention;

[0014]FIG. 3 is a schematic representation of one embodiment of aconstant-current/constant-voltage charging circuit according to theinvention;

[0015]FIG. 4 is one implementation of aconstant-current/constant-voltage charging circuit according to theinvention;

[0016]FIG. 5 is another implementation of aconstant-current/constant-voltage charging circuit according to theinvention;

[0017]FIG. 6 is another implementation of aconstant-current/constant-voltage charging circuit utilizing a currentmirror according to the invention;

[0018]FIG. 7 is another implementation of aconstant-current/constant-voltage charging circuit utilizing a currentmirror according to the invention; and

[0019]FIG. 8 is an embodiment of aconstant-current/constant-voltage/constant-power charging circuitaccording to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0020] Circuitry and methods according to the invention provide a systemthat transitions from constant-current mode—i.e., charging the load at aconstant current—to constant-voltage mode—i.e., continuing to providecurrent to the load while maintaining the voltage across the load at aparticular value.

[0021] A circuit according to the invention preferably provides twopossible charging loops for charging a load. The first loop provides afixed current in order to charge the load. The second loop provides avariable current to charge the load. The variable current variesaccording to the voltage across the load. The variable current is usedto maintain the voltage across the load at a predetermined value.

[0022] The circuit according to the invention receives signals from eachof the first loop and the second loop. The first loop provides a signalthat indicates the fixed current that it is adapted to provide to theload. The value of the fixed current may be determined by the value of afixed or predetermined reference voltage divided by the resistive valueof a resistor. The second loop provides a signal that indicates thelevel of the variable current that the second loop is adapted to provideto the load based on circuit conditions—e.g., the voltage across theload at a given moment.

[0023] The loops are preferably not adapted to provide currentsimultaneously. Rather, each of the loops may preferably provide currentindependently of the other loop—i.e., when the other loop is notproviding current.

[0024] A circuit according to the invention also includes a prioritycircuit. This circuit is configured to receive signals from each of therespective loops. These signals preferably indicate the level of currentthat the particular loop is adapted to provide under present circuitconditions. Thus, the priority circuit is cognizant of the availablefixed current and the available variable current—i.e., that current thatthe second loop provides at the given circuit condition—across theentire operational range of the circuit.

[0025] Then, the priority circuit preferably compares the level ofavailable fixed current to the level of the available variable current.Using the results of the comparison, the priority circuit preferablyselects the smaller of the fixed and the variable current and uses thatsmaller current to charge the load.

[0026] The circuit operates as follows: First, the fixed current, whichpreferably remains substantially constant across the operational rangeof the circuit, is configured to be less than the maximum availablevariable current. At turn-ON of the circuit during, for example, thecharging of an uncharged battery, the priority circuit compares thefixed current to the variable current. Because the battery is notcharged, the variable current request signal is for maximum current. Bydesign, this maximum current is greater than the fixed current requestand the priority circuit selects the fixed current to charge the batteryand begins charging the battery with the fixed current. Thereafter, thevoltage across the battery increases.

[0027] When the battery approaches a state of being fully charged, thevariable current request signal indicates a decrease in the variablecurrent that the second loop is adapted to provide. When the variablecurrent request signal indicates that the requested or availablevariable current is less than the fixed current, the priority circuitbegins using the variable current to charge the battery. At this pointin the charging cycle, the circuit is maintaining the voltage across thebattery at a particular level. Thus, a constant-current/constant voltagesystem, as defined previously, has been established.

[0028]FIG. 1 shows an example of prior art constant-current modecharging schemes that use a programming resistor 110 to set the chargingcurrent. Voltage-controlled current sources 120 and 130 are wired in amaster-slave configuration with the slave 130 supplying the outputcurrent to the load. The master current source 120 is varied such thatV_(ref) appears across resistor 110. This causes a current equal toV_(ref)/resistor 110 to flow in source 120 and, thus, the current equalto V_(ref)/resistor 110 to flow in source 130, and, thereafter, to bedelivered to the load. Resistor 110 is commonly referred to as theprogramming resistor. Amplifier 140 provides the feedback from thevoltage across resistor 110 in order to set the current through source120.

[0029]FIG. 2 shows an example of a constant-voltage mode chargingcircuit in dotted line 215. Amplifier 240 servos the drive voltage tothe current sources 120 and 130 such that a fixed voltage, V_(ref), isdeveloped across the load, Z₁. This circuit ensures that the currentdelivered to the load is varied in order to maintain a constant voltageacross the load. The portion of the circuit within dotted line 225 isnot part of the constant-voltage mode charging scheme. Rather, it isincluded to show the logical progression from conventional circuitry toan embodiment of a circuit according to the invention. Furthermore, inthe embodiment shown in FIG. 2, portion 225 shows that because thecurrent through source 120 is equal to the current through source 130,the voltage developed across resistor 110 is proportional to the currentbeing delivered to the load, Z₁. Thus, FIG. 2 illustrates thepossibility that two charging schemes—i.e., a constant voltage scheme215 and a constant current scheme 225—can coexist.

[0030]FIG. 3 shows a schematic representation 300 of one embodiment of acircuit according to the present invention. Priority circuit 310 has twoinputs, A and B, and an output, Out. Circuit 310 preferably connects thelower of the two inputs, A or B, to the output, Out. Therefore, thecurrent flowing in 120, and thus 130, is equal to either the currentrequired to develop V₁ across resistor 110 or the current required todevelop V₂ across the load, Z₁, whichever current is lower in magnitude.Voltage node, V₃, also preferably continuously provides informationregarding the magnitude of the charging current at all times. In analternative embodiment of the invention, the higher magnitude current,or current identified by another identifying characteristic, may beselected to charge the load.

[0031] In the exemplary embodiment shown in FIG. 3, the operatingconditions of circuit 300 are as follows. The load is a dischargedbattery, V₁/resistor 110 is equal to the desired charging current, andV₂ is equal to the desired final float potential of the battery. Whencharging begins, V₄, the voltage across the battery, is much lower thanV₂, and the output of amplifier 240 slews to the positive supply railbecause amplifier 240 is requesting maximum current.

[0032] Substantially simultaneously, amplifier 140 indicates the voltagethat is necessary to develop V₁ across resistor 110. The voltagenecessary to do this is lower than the positive supply rail (when thevoltage controlled current source 120 is adapted to supply V₁/resistor110 using a control voltage less than the positive supply). Then, thepriority circuit connects the output of amplifier 140 to the controlvoltage of the current sources and ignores the output of amplifier 240.This request causes the current V₁/resistor 110 to be delivered to theload, Z₁. Circuit 300 then behaves exactly like the circuit in FIG. 1.

[0033] As the battery charges and V₄ approaches V₂, the output ofamplifier 240 begins to drop. When the battery voltage, V₄, reaches V₂,the current required by the load to maintain this voltage begins to dropbelow V₁/resistor 110. Amplifier 140 tries to force V₁/resistor 110 intothe battery, but this causes V₄ to rise above V₂ which causes the outputof amplifier 240 to fall quickly. The drop in the output of amplifier240 causes the priority circuit to choose the output of amplifier 240 asthe controlling voltage for the current sources. At this point, theoutput of amplifier 140 is ignored and the loop behaves exactly likecircuit 215 in FIG. 2. The current required by the load to maintainV₄=V₂ is less than V₁/resistor 110, so the voltage across resistor 110,labeled V₃, falls below V₁ and the output of amplifier 140 slews to thepositive rail, and the priority circuit continues to select the constantvoltage loop to provide current to the load. In summary, the currentdelivered to the load is preferably equal to V₁/resistor until thevoltage across the load reaches about V₂. Then, the current delivered tothe load is reduced in order to maintain V₂ across the load. Thiscompletes the constant-current/constant-voltage charging cycle.

[0034] A possible implementation of this invention is shown in FIG. 4.PMOS transistors 410 and 420 function as the voltage controlled currentsources. Two diodes 430 and 440 and a pull-down current source 450perform a diode-or function to implement the priority circuit.

[0035] Circuit 400 shown in FIG. 4 operates as follows. PMOS transistors410 and 420 preferably have a polarity which is opposite the polarity ofvoltage-controlled current sources 120 and 130 shown in FIG. 3. Inaddition, it is well known in the art that increasing gate voltage of aPMOS transistor, while holding the source fixed, decreases thedrain-source current of a PMOS transistor. It follows that, whereasvoltage-controlled current sources 120 and 130 provided higher currentin response to a higher voltage, PMOS transistors 410 and 420 providelower current in response to higher voltage. Furthermore, amplifiers 140and 240 are connected in opposite polarity from the amplifiers 140 and240 shown in FIG. 3.

[0036] In the constant current phase of circuit 400, when the voltageacross the load is less than V₂, amplifier 140 sets the current to theload at V₁/resistor 110. The output of amplifier 140 is preferably thevoltage required to force the non-inverting input of amplifier 140 tohave a voltage V₁. During this constant current phase of the circuit,the output of amplifier 240 is at the negative rail voltage. Thisnegative rail voltage at the output of amplifier 240 is prevented fromaffecting the gate voltage of PMOS transistors 410 and 420 by diode 440.Therefore, the output of amplifier 140 controls the current to the loadduring this phase.

[0037] In the constant voltage phase of the circuit 400, when thevoltage across the load is preferably at or above V₂, amplifier 240 setsthe current to the load such that this current is preferably less thanV₁/resistor 110. During this constant voltage phase of the circuit, theoutput of amplifier 140 is at the negative rail voltage. This negativerail voltage at the output of amplifier 140 is prevented from affectingthe gate voltage of PMOS transistors 410 and 420 by diode 430.Therefore, the output of amplifier 240 controls the current to the loadduring this phase.

[0038] It has been shown that whichever output voltage from amplifiers140 and 240 is higher controls the current to the load. Thus, onefunction of diodes 430 and 440 and PMOS transistors 410 and 420 is toselect the higher output value of amplifiers 140 and 240 to provide thelower available or requested current to the load. Pull down currentsource 450 sets the base-line voltage of the gates of PMOS transistors410 and 420 to zero so the higher output of the amplifiers can be usedto accurately set the voltage of the gates.

[0039]FIG. 5 shows another possible implementation of the invention. Incircuit 500, amplifiers 140 and 240 drive common-source PMOS stages 510and 520. PMOS stages 510 and 520 share a pull-down current source 450just as in circuit 400. In this configuration, however, the output ofamplifiers 140 and 240 is being prioritized by PMOS stages 510 and 520instead of by diodes 480 and 490 (shown in FIG. 4). PMOS stages 510 and520 operate as follows to control the outputs of the amplifiers.

[0040] During the constant current charging phase, amplifier 240 (whichhas its inputs connected in a reverse polarity from FIG. 4) causes theoutput of amplifier 240 to slew to the positive voltage rail. Thiseffectively shuts PMOS stage 510 OFF. Amplifier 140 (which also has itsinputs connected in a reverse polarity from FIG. 4), on the other hand,provide a lower output than amplifier 240 because its inverting input ispreferably lower than the positive supply rail. In this manner, theoutput of amplifier 140 causes PMOS stage 520 to provide the gatevoltage signal at PMOS transistor 410 required to develop V₁ at theinverting input of amplifier 140. This gate voltage signal creates afixed current through PMOS transistors 410 and 420.

[0041] When the voltage across the load is preferably greater than orequal to V₂, the output of amplifier 240 begins to drop. This is similarto the operation of circuits 300 and 400 shown in FIGS. 3 and 4. At thispoint, PMOS stage 510 is turned ON and its drain-source current beginsto control the operation of PMOS transistors 410 and 420. Thisdrain-source current is higher than the drain-source current oftransistor 520 and, therefore, determines the gate voltage oftransistors 410 and 420. When the drain-source current of transistor 510drives the gate voltage of transistors 410 and 420 higher, this causes alower drain-source current in transistors 410 and 420. At this point, aconstant-voltage phase of circuit 500 is commenced and the drain-sourcecurrent in transistors 410 and 420 is varied to maintain a constantvoltage at the load.

[0042] In order to improve the accuracy of the circuit architectureaccording to the invention when low output impedance current sources,such as transistors 410 and 420, are used, a third amplifier 620 can beinserted as shown in FIG. 6. In this circuit, amplifier 620 servos—i.e.,feeds back a signal to—the gate of PMOS transistor 610 such that V_(DS)Of PMOS transistor 410 is equal to V_(DS) of PMOS transistor 420. Whenthese two voltages are equal, the drain-source current of PMOStransistor 420 more precisely mirrors the drain-source current oftransistor 410. It follows that the current through resistor 110 willalso more precisely reflect the drain-source current of transistor 410in this configuration. In order for this circuit to function,I_(load)*resistor 110 should preferably be less than the voltage acrossthe load because the only adjustment that can be implemented throughtransistor 610 is to increase the drain voltage, thereby reducing thedrain-source current through transistor 610.

[0043]FIG. 7 shows another possible embodiment, circuit 700, of thecircuit according to the invention. In circuit 700, amplifier 740provides a current mirror function, together with BJT 750 and currentsource 450 (which is used to pull down the base of transistor 750),between current source resistors 710 and 720. Just as in FIG. 3,voltage, V₃, is preferably proportional to the load current during allphases of circuit operation. Diodes 760 and 770 are used to prioritizethe outputs of amplifiers 140 and 240. Thus, amplifiers 140 and 240operate to control the base current of BJT 750 and, thereby, thecurrents in current source resistors 710 and 720. In this way, operationof amplifiers 140 and 240 is similar to the operation of amplifiers 140and 240 in FIGS. 3-6. Main differences between circuit 700 and thecircuits shown in FIGS. 3-6 include the implementation of controllablecurrent source resistors 710 and 720 and the use of the current mirrorto set the currents through resistors 710 and 720 substantially equal toone another.

[0044] The concept of this invention can be extended to include anynumber of input variables, not just constant-current andconstant-voltage. For example, consider a case where the rate of powerbeing dissipated in the current source driving the load must be limited.Circuit 800 in FIG. 8 includes an example of a priority circuit 840 thatchooses between constant-current, constant-voltage, and constant-powercharging of a load.

[0045] Operation of circuit 800 is exactly like that of FIG. 3, but nowthe output of amplifier 810 is added as an input, to the prioritycircuit 840. If, at any time, the power dissipated in current source 130causes its temperature to exceed T_(ref), then the output of amplifier810 falls low enough so that the priority circuit 840 gives amplifier810 control of the current sources. The operation of circuit 800requires that the power dissipated in source 130 is proportional to thetemperature of source 130. In this condition, amplifier 810 holds source130 at a constant temperature, and thus, charges the load whilemaintaining constant power dissipation in source 130. It should be notedthat the voltage across resistor 110 continues to be proportional to thecharging current in this instance, just like in constant-currentoperation and constant voltage operation. Obvious extensions of FIGS. 4and 5 can be used as possible implementations for the circuit shown inFIG. 8.

[0046] In conclusion, this invention disclosure presents a method ofselecting one of several different feedback loops, used to control thecharging of a load, according to a certain priority. A common example ofa system benefitting from such an invention is the charging of alithium-ion battery.

[0047] Thus, a constant-current/constant-voltage charging circuit isprovided. One skilled in the art will appreciate that the presentinvention can be practiced by other than the described embodiments,which are presented for purposes of illustration and not of limitation,and the present invention is limited only by the claims which follow.

What is claimed is:
 1. A method for providing a current to a load,comprising: selecting a current provided to the load from one of a fixedcurrent, a constant power current, and a variable current, the variablecurrent being based on the voltage across the load, the selectingrequiring selecting one of the fixed current, the variable current, andthe constant power current based on a predetermined priority, thepredetermined priority requiring selection of the current that providesthe lower magnitude current or limits power dissipation of a currentsource providing current to the load.
 2. The method of claim 1, whereinselection of the constant power current causes the current source toemit a predetermined temperature.
 3. The method of claim 1, wherein thepredetermined priority selects the constant power source if the powerdissipation of the current source exceeds a predetermined level of powerdissipation.
 4. A circuit for providing current to a load, the circuitcomprising: a first current loop that is adapted to provide a fixedcurrent; a second current loop that is adapted to provide a variablecurrent; a third current loop that is adapted to provide a constantpower current; and a priority circuit that receives a first signal fromthe first current loop which indicates the level of an available fixedcurrent, a second signal from the second current loop which indicatesthe level of an available variable current, and a constant power signalfrom a constant power loop which indicates the level of powerdissipation of a current source and selects one of the first currentloop, the second current loop, and the constant power loop to providecurrent to the load based on a predetermined priority signal, thepredetermined priority requiring selection of the current that providesthe lower magnitude current or limits power dissipation of the currentsource.
 5. The circuit of claim 4, wherein selection of the constantpower current causes the current source to emit a predeterminedtemperature.
 6. The method of claim 4, wherein the predeterminedpriority selects the constant power source if the power dissipation ofthe current source exceeds a predetermined level of power dissipation.